Currently, aluminum alloys, magnesium alloys and plastic are employed for boards used as assembling components in various types of electronic products such as personal computers, cell phones, digital cameras, LCD (Liquid Crystal Display) televisions, plasma televisions, printers, copiers and other products incorporated with computer systems. This to meet the demands for more compact, lighter, and thinner products; and those boards are tending to become thinner. For this reason, it is necessary, as described above, to use large numbers of small-sized screws in assembling of these products and to quickly, reliably and accurately perform screw-mounting and tightening work. Also, the trend for LCD and plasma TV screens to become larger in recent years has increased the number of screws used since the above-described boards used in them become longer in size.
In order to quickly mount screws when assembling the above-described electronic products, a large number of screw holes are pre-installed at required locations on assembly component boards; and these large number of screw holes are formed simultaneously during the molding and machining works of the boards which are under the conditions described above. Accordingly, it is inevitable that these screw holes experience subtle changes in, for instance, the position and the shape due to the temperature and machining states of the board during the molding and machining work. When misalignment phenomena of the screws occur between, for example, a substrate and a mounting board, the screws may be unable to be screwed smoothly into the substrate when the screws are positioned in the screw holes installed on the mounting board and tightened.
In the past, tapping screws are generally used when products are to be assembled with small-sized screws. More specifically, tapping screws are positioned in place in prepared pilot holes of an assembly component and screwed in, thus fixing the parts while the tapping screws form internal threads in the pilot hole. However, in such cases a difficulty arises that tapping of the internal threads creates frictional heat and molding powders that can cause damages and breakdown in, for example, the electronic parts.
To solve the above-described problems, an internal-thread-forming tapping screw has been proposed, In this internal-thread-forming tapping screw, the pitch of the threads is relatively larger, the groove portions of the screw are formed in a relatively large interval, and at least 60%, starting from the tip end of the threaded portion of the total length of the threaded portion where the thread is formed, is coated with a resin adhesive in which epoxy-type adhesives are encapsulated in microcapsules (see patent document 1).
Also, when putting together assembly components, surface treatments such as painting or plating is occasionally applied after tapping the internal threads to a frame or assembly components. In such a case, the internal threads would become slightly smaller due to the surface treatment, or paint would get into the tapped holes, making it difficult for the male threads of bolts and set screws of corresponding size to enter thereinto. Also, when removing assembly components, internal threads can be damaged or rust may occur on the internal threads when previously-prepared internal threads are subsequently used to put together the original assembly components or new assembly components; and this makes it difficult for the male threads to enter.
A known way to repair internal threads is to use a hand-turning tap when the problems as described above occur with the internal thread. However, this task is extremely troublesome. For that reason, a tap (tool) has been proposed to replace the hand-turning tap. This tap (too) is configured such that a cutting edge is provided on the outer perimeter of the threaded portion, and at least one groove that can eject the cut chips is also provided; and this tap (tool) is screwed into the internal threads to repair internal threads that have become difficult for male threads to enter. Such a repair work can be done very easily with this tool.
Also, when mounting and tightening are done using small-sized screws for the product assembly as described above, studs, spacers, or the like equipped with internal threads are used to ensure heat radiation paths for heat generated in, for instance, electronic components. In these cases as well, the same problems as described above occur when mounting male threads in internal threads.
To resolve the above-described problems, a loose preventive male screw has been proposed. In this screw, the threaded portion is divided into two parts, a screw head portion and a distal end side, to provide a first area of threads and a second area of threads; and the threads in the distal end side is shaped such that the threads gradually increase in outer diameter from the tip end to form a tapping thread that is larger in diameter than the threads in the screw head portion (see patent document 3).    Patent document 1: Japanese Patent Application Laid-Open (Kokai) No. 2002-70824    Patent document 2: Japanese Patent Application Laid-Open (Kokai) No. 2005-254444    Patent document 3: Japanese Patent Application Laid-Open (Kokai) No. 2005-337406